The neural structures which regulate some forms of behavior and those which regulate gonadotropin release undergo a sexual differentiation process during development which appears to be under the control of androgen. The criteria hitherto used to evaluate some aspects of this process have been unsatisfactory. Territorial marking in the Mongolian gerbil is a sexually differentiated behavior which offers unique advantages for investigating sexual differentiation of the brain. Using this new model system as a foundation, studies will be undertaken to (1) characterize the critical period, neural structures and role of gonadal steroids in the sexual differentiation process occurring in the genetic male; (2) compare and contrast the sexual differentiation process in the genetic male and androgenized female; (3) compare the sexual differentiation of the behavioral control system with that of the gonadotropin control system; (4) investigate the mechanism of androgen action in the sexual differentiation process. These studies will involve the manipulation of the sexual differentiation process by paranatal systemic or hypothalamic administration of steroids or pharmacological agents and neonatal gonadectomy. The effects of neonatal manipulations will be determined by analysis of (1) behavioral responses to exogenous hormones and (2) feedback responses of gonadotropin release to exogenous steroids determined by radioimmunoassay of plasma LH in adulthood.